Ref. 5013 Production of this reference started in 1992. This reference was until the introduction of the Sky Moon Tourbillon (Ref. 5002) the most complex wristwatch Patek Philippe produced. Self-winding, its movement consists of 515 parts and shares similarities with Ref. 5016, the manual-winding retrograde date perpetual calendar with tourbillon and 506 parts. Both watches incorporate perpetual calendar functions with a retrograde, or flyback, date hand whose specially developed mechanism absorbs almost no energy when the hand returns on the 1st of each month.Month and year indications in the leap year cycle will change instantaneously and if always kept running, no adjustments need to be made until Feb. 28, 2100 (The moonphase indication will show a deviation of one single day only after 122 years and 45 days). Combining a minute repeater with a retrograde date function certainly sets this highly complex timepiece apart from other watches and a very small stamp on the balance bridge, the "Geneva Seal" or Geneva Quality Hallmark, makes a further impotant difference. It attests, that Ref. 5013 is produced according to the highest standards of watchmaking in the world. A similar watch is published in ?Collecting Patek Philippe Wristwatches?, by Osvaldo Patrizzi, Guido Mondani Editore, 2004 edition, page 277.